The GPS, or other global navigational satellite systems (GNSS), can be used to determine the position of a user on or near the earth from signals received from multiple orbiting satellites, sometimes referred to as space vehicles (SVs). The latitude, longitude, and altitude of any point close to the earth can be calculated from the times of reception of the electromagnetic signals from four or more satellites to the unknown location of a GPS or GNSS receiver. In the GPS satellite constellation, each satellite transmits two spread-spectrum signals in the L-band, known as L1 and L2. The L1 signal is transmitted at a center frequency of 1575.42 MHz, while the L2 signal is transmitted at a center frequency of 1227.6 MHz.
The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is normally encrypted, with the encrypted version of the P-code referred to as the Y-code. The P and Y codes are often referred to as the P(Y)-code. The L2 signal from each satellite is modulated by the P(Y)-code.
Intentional and unintentional jamming of GPS satellite C/A-code and P(Y)-code signals prevents signal acquisition and tracking by GPS receivers. The relatively low power of GPS satellite signals facilitates the ease of their jamming. Terrestrial based (i.e., ground, ship or aircraft based) GPS transmitters, sometimes referred to as pseudolites, can be used to transmit the L1 and L2 signals. These pseudolites can be used to increase the jamming levels at which GPS acquisition and navigation can be achieved by providing much higher signal power than that provided by the GPS satellites. It is feasible to provide pseudolite signal power levels 30 to 60 dB greater than GPS satellite signal power levels. The result is that the adversary needs to develop 30 to 60 dB greater jammer power to jam the pseudolite signals than is required to jam the GPS satellite signals.
Pseudolites and their higher power GPS signals provide certain disadvantages or problems. For example, the high power GPS pseudolite C/A signals can cross-correlate with the lower power GPS satellite signals, thereby denying the normal GPS signal acquisition and C/A code tracking. Further, the GPS pseudolite signals from one pseudolite can actually cross-correlate with the signals from another more distant pseudolite. This unintentional signal cross-correlation jamming is sometimes referred to as the near/far problem. Additionally, the high power pseudolite signals act as broad band jammer signals when attempting to track the low power GPS satellite signals, thereby jamming the low power GPS satellite signals.